Sure hate to see you suffer with that old decal. I'll give you ten bucks for the decal and the bike, and then the decal problem will be off your shoulders.
Post a photo of the flip side of the decal. Not sure from the description which side is "plastic" and which is paper. The matrix that holds the decal is the 3M label side in the first photo, and it is probably some form of synthetic fabric or paper fiber product. It may have a very thin plastic skin or may be entirelynplasticized. The backing, which should,be the side not shown in the first photo is probably heavier paper with a plastic skin that the decal isn't supposed to stick to, but the matrix will stick to. They should just barely stick together, and should separate. When the upper matrix is lifted from the paper backing the numbers should stay with the upper matrix. That's how I would understand this thing to work, and there should be three layers: upper matrix, vinyl decal, paper backing.
You probably already know all this, but since it doesn't seem to want to work right, try warming the decal before trying to separate the thing. Somewhere around 100 to 120 degrees. See if that helps things function.